Field emission cathode

ABSTRACT

A field emission cathode comprises a glass substrate in combination with an insulating layer sandwiched between two conducting layers functioning as electrodes. Cavities extending through one of the electrodes and the insulating layer to the surface of the other electrode contain highly compacted and porous electron emissive layers which provide high electrical resistance between said electrodes and are diffused with metallic gold to form needle-shaped metal deposits on the surface as well as within the emissive layers. Such cathodes are used in image converters and image display tubes.

United States Patent 1191 1111 3,735,186 Klopfer et a1. May 22, 1973 1541 FIELD EMISSION CATHODE OTHER PUBLICATIONS Inventors: Anton Martin p Aachen; C. A. Spindt, A Thin-Film Field-Emission Cathode,

Georg Dittmer, 5101 Roetgen, both of Germany U.S. Philips Corporation, York,N.Y.

Filed: Mar. 10, 1971 Appl. No.: 122,851

Assignee: New

Foreign Application Priority Data I Mar. 14, 1970 Germany ..P 20 20 121.9

References Cited UNITED STATES PATENTS 3/1970 Crost et a1. ..3l3/309 X J. Applied Physics, Vol. 39, No. 7, June 1968, pp. 3504-3505.

Primary Examiner--Ronald L. Wibert Assistant Examiner-Paul A. Sacher Attorney-Frank R. Trifari [57] ABSTRACT A field emission cathode comprises a glass substrate in combination with an insulating layer sandwiched between two conducting layers functioning as electrodes. Cavities extending through one of the electrodes and the insulating layer to the surface of the other electrode contain highly compacted and porous electron emissive layers which provide high electrical resistance between said electrodes and are diffused with metallic gold to form needle-shaped metal deposits on the surface as well as within the emissive layers. Such cathodes are used in image converters and image display tubes.

7 Claims, 8 Drawing Figures mas/Q Patented May 22, 1973 3,735,186

2 Sheets-Sheet l Fig.1 Fig.2

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7 V ,F|g 6 INVENTORB GEORG DITTMEAR Patented May 22, 1973 2 Sheets-Sheet 2 Fig.7

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INVENTORS A NTON M. KLOPFER BY GEORG DITTMER AGENT FIELD EMISSION CATIIODE The invention relates to a cathode assembly for an electric discharge tube in which an insulating layer is sandwiched between two conductive layers functioning as electrodes.

From US. Pat. Specification No. 3.184.659 such a cathode is known. An aluminum substrate is cdvered by a vitreous alumina layer of 75 A thickness at the most and on this layer a 1500 A silicon layer is deposited from the vapour phase, With the aid of a mask a circular opening of about 0.25 mm is recessed in the vapour-deposited layer. The edges of the opening and the vapour-deposited layer itself are covered by a 600 A palladium layer. The alumina exposed in the opening is covered by a 25 A platinum layer, which extends on the edges of the palladium layer. A crosswise system of palladium strips having a thickness of 600 A, a width and an interval between them of 1, each is vapor deposited on the platinum layer in the opening. A caesium mono atomic layer is superimposed. As an alternative, the palladium strip systemm'ay be omitted. The maximum permissible voltage across the alumina layer is then, however, only 2 V, whereas it is 9 V, when the palladium strips are used. The current density across the apparently emissive surface is then 0.13 mA/cm and 8 mA/cm respectively. In the latter case the lifetime is said to be more than 400 hours. However, the resultant current densities are too low for the majority of uses. In accordance with the invention a cathode is formed by a layer insulating material sandwiched between two conductive layers functioning as electrodes. The insulating layer and one of the conductive layers have cavities therein which extend into the other conductive layer and which have a maximum diameter not exceeding 50 i.e. are surrounded by material of diminishing thickness. The cavities are filled with a highly-compacted electron emissive material on which a high resistance electron emissive layer is deposited. A gold layer is partly diffused in the form of needleshaped metal deposits into the high resistance electronemissive layer.

The forming process consists essentially in a diffusion of the metal film located in the active cavity rangeso'n the porous insulating layer. In the grain boundaries of the porous insulator the metal settles in the form of field-emissive micro-regions. The subjacent, compact insulating layer prevents complete diffusion of the immigrating metal down to the base electrode. Moreover, the store of metal available for the post-diffusion is exhausted after the formation owing to the thin metal cover so that a stationary state can be established during the further hours of operation. The effect of the electric field in the range of the non-coherent metal layer in the emissive region is concentric in accordance with the geometrical conditions. Accordingly the diffusion of the metal is enlarged progressively at the edge of the emissive region. The potential distribution in the emissive region adjusts itself in such a way that a large portion of the applied voltage extends over a layer in the proximity of the surface bounding the vacuum. At this place the greatest density of the emissive metal micro-regions are found in the porous insulator. Such a structure has the following advantages.

First the potential is applied unattenuated via the base electrode to the emissive region. In the emissive regions themselves the metal coating layer is interrupted so that the emitted electrons are not subjected to additional energy losses in the coating electrode. Field-emitting metal deposits are protected against electrical break-down, since in the event of an abruptly increasing field emission the subsequent supply of electrons is limited due to the porous insulating layer operating as a bias resistor and due to the limitation of the injection of electrons into the porous layer through the compact insulating layer.

Since the transverse dimensions of the emissive regions are smaller than u, the high resistivity of the covering layer in these regions has no adverse effect. In order to avoid too high an electric resistance of the covering electrode, it may be thicker between the cavities of the insulating layer and may preferably be formed by a layer of less readily diffusing metal and by a superimposed layer of more readily diffusing metal.

The lower conductive layer may be a metal layer or a photo-conductive layer so that the cathode is suitable for use in image converters.

A discharge tube according to the invention may be constructed in the form of an image display tube of flat structure, in which the two electrodes are formed by a system of crossing strips.

In a method of manufacturing a cathode for use in a discharge tube embodying the invention a support is covered by a conductive basic layer, on which first an insulating layer is arranged in a thickness of about 1 u. The latter layer has vapor deposited on it a covering electrode of a metal not readily capable of diffusing. The electrode is covered by photo-lacquer, in which holes of a diameter of at the most 50 u are developed.

Subsequently, the covering electrode and the insulating layer are etched down to the basic layer and the photolacquer is chemically removed. The assembly is subsequently oxidized thermally, the exposed portions of the basic layer and the perforated covering electrode being both covered by a dense oxide layer. Then only the basic layer is anodically oxidized in a manner such that a porous oxide layer is formed in the holes of the oxide layer already available and on the oxide layer obtained thermally. The next step is the vapour-deposition of a gold layer. The cathode is formed in high-vacuum at 300 C and 9 V potential difference between the electrodes, the covering electrode being positive. After mounting in a discharge tube the cathode can be degassed at 350 C without applying a voltage.

In a further method first the basic layer, the insulating layer and the covering electrode are made, as well as the photo-lacquer layer with developed holes. Subsequently, the holes are etched through down to the basic layer and both the dense insulating layer and the porous insulating layer are provided in the holes by vapour-deposition or cathode-sputtering. This method is particularly suitable, when the basic layer is photoconductive and cannot be oxidized in some way or other. It is then advisable to perform the etching operation by ion bombardment, since no contamination can be caused by chemical solutions, while at the same time the photo-lacquer is removed. The shape of the etched holes is then better than in a chemical etching process, since in the latter case strong under-ething takes place.

The invention will be described more fullywith reference to the drawing, in which FIGS. 1 to 6 illustrate various states corresponding to separate steps of a method of manufacturing a cathode according to the invention.

FIG. 7 is a sectional view of an image converter employing a cathode according to the invention and FIG. 3 is a plan view of a cathode according to the invention in a display tube.

Referring now to FIG. 1 reference numeral 1 designates a glass substrate, on which an aluminum layer 2 of 1000 A is vapor deposited; 3 designates an SiO layer of lp. thickness applied by sputtering; 4 designates the covering aluminum electrode of 1000 A thickness; 5 denotes a photo-lacquer layer having holes of a diameter of 50 u and spaced apart from each other by 50 ,u.. Only one hole 6 is shown. FIG. 2 shows the hole 6 etched down to the aluminum layer 2.

After the chemical removal of the photo-lacquer layer the assembly is subjected to thermal oxidation so that an A1 layer of 50 A thickness 7 of high compactness is formed in the hole 6 on the aluminum layer 2. The A1 0 layer 8 is formed on the perforated covering electrode 4 (FIG. 3).

in a mellitic acid solution the thermal oxide layer 7 is covered at a high current density by a porous A1 0 layer 9 (compactness about 40 to 50 percent of the maximum compactness) (see FIG. 4).

Subsequently (as is shown in FIG. a gold layer 10 having a thickness of 50 A is vapor-deposited. The cathode is heated in high vacuum (better than 10' Torr) at 300 C with the covering electrode at a potential of 9 V positive with respect to the electrode 2, for 4 hours.

The gold on the oxide layer 9 diffuses partly in the form of substantially needle-shaped metal deposits 11, whereas portions 12 remain on the surface. The gold layer 10 on the oxide layer 8 diffuses in the latter, but it is not of any interest (FIG. 6). Among the Figures, which are not to scale particularly with respect to the ratio between the diameter and the depth, of the holes 6, FIG. 6 is a slightly enlarged view. The cathode is capable of supplying, at 12 to l5 V between the electrodes, a current from the apparently emitting surface of about 100 mAlcm The lifetime is more then 1000 hours.

Referring to FIG. 7, reference numeral 21 designates a glass plate provided with a transparent SnO layer 22 of 200 A; reference numeral 23 is a photo-conductive layer of 1500 A; 24 denotes an SiO layer of 1 p. applied by sputtering. Holes 25 are provided in the layer 24 and in the vapor-deposited covering electrode 30. For marking the holes a photo-lacquer layer having holes of 50 diameter had been provided. In the holes 25 first a solid SiO layer 26 of 100 A is applied by sputtering, on which a porous layer 27 of SiO (x being about 1.4 to 1.5) is applied to a thickness of 500 A. Gold is vapordeposited to a thickness of 50 A and diffused at 300 C, while a voltage is applied. Gold deposits in the layer 27 are denoted by 28 and residual gold on the surface is denoted by 29. All deposits on the covering electrode are designated by 31, but they are of no importance.

At a distance of 2 cms a phosphor screen is provided, which consists of a thin aluminum layer 32, a phosphor layer 33 and a galss support 34. The two glass supports 21 and 34 form part of the vauum envelope. When the cathode receives a voltage of 15 V between the electrodes and the phosphor screen receives a few kilovolts relative to the cathode, an image projected onto the cathode can be displayed on the phosphor screen in an intensified state.

Referring to FIG. 8, reference numeral 40 designates an insulating base plate having vertical base electrodes 42 in the form of strips and horizontal electrodes 41. At the crossings the upper electrodes are perforated as described with reference to FIGS. 1 to 6. Separate holes are designated by 43. When the strips have a width of l mm, each crossing has about hundred holes. When a phosphor screen is arranged in front of this cathode and the cathode is correctly excited, a picture can be obtained on the screen.

What is claimed is:

l. A cathode assembly for an electric discharge tube comprising an insulating layer, a pair of electrodes comprising conductive layers separated by said insulating layer, a plurality of cavities extending through one of said conductive layers and said insulating layer into said other conductive layer and being surrounded by material of diminishing thickness, each of said cavities having a maximum diameter not exceeding 50p, a highly-cornpacted electron emissive layer within each of said cavities, a high-resistance electron emissive layer on said highly compacted layer, and a gold layer deposited on said high resistance electron emissive layer, said gold layer being partly diffused in the form of substantially needle-shaped metal deposits within and on the surface of said high resistance electron emissive layer.

2. A cathode assembly as claimed in claim 1 wherein said electrodes comprise aluminum layers having thicknesses of about 1000 A.

3. A cathode assembly as claimed in claim 1 wherein said insulating layer comprises SiO having a thickness of lp..

4. A cathode assembly as claimed in claim 1 wherein said cavities are spaced apart approximately 50p. from each other.

5. A cathode assembly as claimed in claim 1 wherein said highly compacted electron emissive layer A1 0 have a thickness about 50 A.

6. A cathode assembly as claimed in claim 1 wherein said high resistance electron emissive layer comprises a layer of A1 0 having a compactness of about 40 to 50 percent of the maximum compactness of this material.

7. A cathode assembly as claimed in claim 1 wherein said gold layer has a thickness of about 50 A.

UNITED STATES PATENT OFFICE r CERTIFICATE OF CORRECTION Patent 2 735.186 Dat Maw 22, 1973 Inventor-(s) ANTON M. KLOPFER AND GEORG DIT'I'MER It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Change Priority No. from "p 20 20121.9" to read P. 20l2l92.0-.

Signed and sealed this 29th day of 0ctober 1974.

(SEAL) Attest: v I

c. MARSHALL DANN Commissioner of Patents McCOY M. GIBSON JR. Attesting Officer 

2. A cathode assembly as claimed in claim 1 wherein said electrodes comprise aluminum layers having thicknesses of about 1000 A.
 3. A cathode assembly as claimed in claim 1 wherein said insulating layer comprises SiO2 having a thickness of 1 Mu .
 4. A cathode assembly as claimed in claim 1 wherein said cavities are spaced apart approximately 50 Mu from each other.
 5. A cathode assembly as claimed in claim 1 wherein said highly compacted electron emissive layer Al2O3 have a thickness about 50 A.
 6. A cathode assembly as claimed in claim 1 wherein said high resistance electron emissive layer comprises a layer of Al2O3 having a compactness of about 40 to 50 percent of the maximum compactness of this material.
 7. A cathode assembly as claimed in claim 1 wherein said gold layer has a thickness of about 50 A. 